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Basic Ultrasound Physics

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  1. Section 1

    What is Ultrasound ?
  2. How Are Images Generated ?
  3. Ultrasound Physics Videos
  4. Section 2
    Artifacts
  5. Section 3
    Probe Characteristics
  6. Knobology
  7. Imaging Modes
  8. Section 4
    Scanning & Display Orientation

Quizzes

Basic Ultrasound Physics Artifacts
Lesson 4 of 8
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Artifacts

Introduction

Artifacts are display images that do not correlate with true anatomical structures. Artifacts can cause the appearance of structures that are not anatomically present to appear on the display. Conversely, structures which are present anatomically may be completely absent on the display. Artifacts may also cause anatomical structures to be visualized in an incorrect location, size or brightness.

This can cause confusion in image interpretation. Knowledge of artifacts and why they happen can help us to avoid pitfalls in diagnosis and improve imaging quality. We hope you find this summary of what is a difficult topic useful !

Basic Assumptions

There are several assumptions made by an ultrasound machine during image processing and display. These include:

1.      Sound travels in a straight line through tissues. Sound beams travel directly to a reflector and back to source.

2.      Reflections are only produced by structures along the beams main axis.

3.      Sound travels at 1540 m/s regardless of tissue type. Therefore the time taken for a reflected ultrasound beam to return to the transducer determines the depth of the reflector.

4.      There is uniformal attenuation of the ultrasound acoustic energy regardless of tissue type.

5.      The ultrasound beam is infinitely thin and 2 dimensional.

Commonly Encountered Artifacts

Artifacts occur because these basic assumptions of ultrasound imaging are violated. Some artifacts may hinder imaging, whilst others may lead to potential false diagnosis. In some organs such as the lung, artefacts play an important role in detecting pathology. Examples of artifacts:

  1. Mirror Imaging Artifact
  2. Acoustic Shadowing Artifact
  3. Posterior Acoustic Enhancement
  4. Edge Shadowing Artifact
  5. Reverberation Artifact
  6. Comet Tail Artifact
  7. Ring Down Artifact
  8. Side Lobe Artifact

Classification of Artifacts

Artifacts can be calssified into:

  1. Artifacts related to ultrasound beam properties.
  2. Artifacts related to wave reflection or refraction.
  3. Doppler artifacts – these are not covered presently.

a) Side Lobe Artifact

Fig 1.0. The ultrasound beam consists of a main beam that is ‘bow tie’ shaped. There are also side lobes and grating lobes that arise from the transducer radially. Image from: US Artifacts. Feldman et al. RadioGraphics 2009.

When a side lobe beam encounters a STRONG REFLECTOR, the reflected side lobe beam will be interpreted by the processing functions of the machine as a reflection from the main beam (where the transducer is ‘looking’). This results in the display of the reflector along the path of the main beam instead of its anatomical location.

Fig 1.1. (B) PLAX view with linear side lobe artifact (arrow) in the ascending aorta due to a calcified sinotubular junction (arrowhead). (C) PLAX view of a healthy patient with strongly reflecting pericardium, causing a side lobe artifact in the left atrium (arrow). Image from: Fact or Artifact in Two-Dimensional Echocardiography: Avoiding Misdiagnosis and Missed Diagnosis. Bertrand et al. Journal of the American Society of Echocardiography 2016.

b) Beam Width Artifact

This is caused by the violoation of the assumption that the ultrasound beam is infinitely thin. In reality, the lateral and elevational width of the beam give the ultrasound beam a 3-Dimensional shape. Anatomical structures outside of the 2D imaging plane are then incorrectly positioned as if they are within the scanning plane on the display monitor.

c) Near Field Clutter

Structures in the near field are sometimes obscured due to the high amplitude of oscillations by the transducer itself, causing a so called “near field clutter”.

a) Mirror Image

Mirror Image Artifact
Fig 1.2. The liver is mirrorred in the lung above the diaphram (arrow), a strong reflector. Dwyer, Kristin & Rempell, Joshua. (2016). Young Woman with a Fever and Chest Pain. Western Journal of Emergency Medicine. Image available via license: Creative Commons Attribution 4.0 International

A mirror image artifact typically appears below a strong reflective surface producing a duplicate image behind the mirror of the real structures in front of the mirror.

b) Acoustic Shadowing

Fig 1.3. Ultrasound beam does not penetrate strong reflectors, resulting in shadowing. Creative Commons License: Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0). Image courtesy of Dan O’Brien.

Acoustic shadowing results in the absence of echoes behind a STRONG reflector. This is due to a strong reflector or refractor preventing ultrasound wave propagation beyond it.

c) Acoustic Enhancement

Fig. 1.4. Acoustic enhancement or posterior enhancement refers to increased echoes deep to structures that transmit sound exceptionally well. Image with permission from A.M Marjan. Basic Concepts in Ultrasound Physics.

When the ultrasound beam encounts a weakly attenuating structure, the echoes received from points distal to this material are higher in intensity than echoes received from a similar depth in the imaging plane. This results in a brighter display of the image distal to the weak attenuator. This is characteristic of fluid-filled structures such as cysts and the gallbladder.

d) Edge Shadowing

Fig 1.5. Edge artifacts are caused by scattering and refraction of sound energy by a curved reflector. This leads to reduced intensity of the reflected ultrasound beam returning to the transducer, causing a shadow appearance. Image with permission from A. M Marjan, Basic Concepts in Ultrasound Physics.

e) Reverberation Artifact

A lines
Fig 1.6. Reverberation artifacts – A Lines from an ultrasound scan of the lungs.

These are ghost images caused by reflection of ultrasound waves between 2 strong reflectors. This causes delayed return of the beam to the transducer leading the machine to interpret the presence of multiple ghost images at equidistant intervals below the true image. A Lines seen in a well aerated lung are a classic example. These are caused by repeated reflection of the ultrasound beam between the pleural surface and the trasnducer head.

Reverberation artifacts can be reduced or avoided by reducing the gain or scanning at different angles. A lines seen in lung ultrasound signify well aerated lungs. They are reverberation artifacts that will not be seen if the angle between the ultrasound beam and pleural surface is not perpendicular.

f) Comet Tail

Fig 1.7. Comet tail artifacts seen at the pleural surface. These are known as z-lines. They are ill-defined, short, and do not erase A-lines and do not move with respiration. They are insignificant findings. Comet tail artifacts are short path reverberation artifacts.

Fig 1.8. Comet tail artifacts are also seen in places such as the descending aorta where they can be seen to radiate from the distal wall. They are caused by closely spaced reflectors, in the case of the aorta – this is due to aortic wall calcification. Image from: Imaging Artifacts in Echocardiography. Le et al. A&A 2016.

g) Ring Down Artifact (B Lines)

B lines
Fig 1.9. Example of ring down artifact. These are B-lines, seen in lungs when the alveoli are fluid filled. They are not the same as comet tails, although the 2 labels are wrongly used interchangeably at times. B-Lines will obscure A-Lines.

Small pockets of fluid trapped by surrounding air bubbles can cause ring down artifact. When ultrasound waves hit a pocket of trapped fluid, the sound waves resonate within the fluid before being transmitted back to the transducer. The resonant vibrations detected by the ultrasound transducer are displayed as bright vertical lines extending deep to the trapped fluid.

Video: Ultrasound Artifacts

Video from: American Thoracic Society: Ultrasound Artifacts: Authors Sachita Shah, M.D. Amy Morris, M.D. Manjiri Dighe, M.D. Institution University of Washington

Further Reading

  1. Imaging Artifacts in Echocardiography. Le et al. Anesthesia & Analgesia 2016.
  2. Fact or Artifact in Two-Dimensional Echocardiography: Avoiding Misdiagnosis and Missed Diagnosis. Bertrand et al. JASE 2016.
  3. US Artifacts. Feldman et al. RSNA. 2009